if (/($regex_string)/is) {

##</code><code>##

while (/$regex_string)/isg) {
    print "\nmatched in $arg1:\n==$1==\n";
}

##</code><code>##

part1.*?part2	part1.*?part2
Foo bar
Other pattern

##</code><code>##

hghghgghghh
part1
fff
part2
blah blah
Foo bar
more noise
Other pattern
another match for
part1
stuff I
don't want
to see part2
blabla

##</code><code>##

#!/usr/bin/perl -w

use strict;

my $patterns = "patterns.txt";
my $arg1 = shift;
my $regex_string = '';
my %patterns;

open (PAT, '<', $patterns) or die "$patterns: $!\n";
while (<PAT>) {
    chomp;
    my ( $pattern, $replace ) = split '\t';
    $patterns{$pattern} = $replace if ( $replace );
    $regex_string .= $pattern . '|';
}
close(PAT);

chop $regex_string;  # remove string-final '|';

open( FILE, "<", "$arg1") or die "$arg1: $!\n";
$_ = do { local $/; <FILE> };
close(FILE);

while ( /($regex_string)/gis ) {
    my $match = $1;
    for my $replc ( keys %patterns ) {
        last if ( $match =~ s/$replc/$patterns{$replc}/is );
    }
    print "\nmatched in $arg1:\n==$match==\n";
}